This invention relates to a process for the preparation of easily dispersible pigments of the beta-phase modification of metal phthalocyanine pigments, especially that of copper phthalocyanines. This process provides a simple and cost-effective method for manufacturing soft green-shade blue pigments having high color strength.
Crude copper phthalocyanine is usually prepared by a reaction of phthalic anhydride or a derivative thereof, urea, and a copper source, or by a reaction of phthalonitrile or a derivative thereof and a copper source in an organic solvent, optionally in the presence of ammonium or titanium salts (such as titanium tetrachloride). The resultant phthalocyanine particles, however, undergo crystal growth during preparation and have a major axis of about 10 to about 200 .mu.m. Such copper phthalocyanines have little or no color value as a pigment for use in inks, coating compositions, plastics, and the like. For this reason, the crude copper phthalocyanine must be milled to obtain a high color value. Pigment milled in this way has a particle size of about 0.01 to about 0.5 .mu.m and is a mixture of alpha and beta phases. The milled pigment is then recrystallized (or "finished") in a mixture of water and an organic solvent to provide optimum color properties of the beta-phase green-shade blue pigment.
Several methods have been described for preparing pigments having improved color value but these methods typically produce mixtures of alpha- and beta-crystal forms or require the use of special additives or solvent mixtures.
For example, British Patent 1,502,884 discloses a one-step process for preparing dispersible pigments of the beta-modification of copper phthalocyanine in which a coarser form of highly purified beta-phase copper phthalocyanine is ground in an organic liquid (optionally containing water) in the presence of certain phthalocyanine derivatives that improve dispersibility of the pigment.
U.S. Pat. No. 4,104,276 discloses the use of certain copper phthalocyanine derivatives as additives to inhibit "overcrystallization" when milled phthalocyanine pigments are recrystallized in organic or aqueous organic solvents.
U.S. Pat. No. 4,104,277 discloses a process for preparing an alpha-phase or beta-phase copper phthalocyanine 10, pigment. A crude copper phthalocyanine is first dry milled (which increases the alpha phase relative to the beta phase) and then mixed with an emulsion containing an amine, water, and a surfactant.
U.S. Pat. No. 4,158,572 discloses a method of making a pure beta-phase phthalocyanine pigment. An initial dry milling operation produces a mixture of alpha- and beta-crystal forms. The mixture is then conditioned by stirring in an aqueous medium containing a surfactant to obtain the required pigment particle size and to convert the remaining alpha-phase phthalocyanine to the beta phase.
U.S. Pat. No. 4,257,951 discloses a two-step milling process for preparing beta-phase copper phthalocyanine pigment. In the first stage, a crude copper phthalocyanine is dry milled in the presence of a hydrated aluminum sulfate and an organic solvent. In the second stage, the milled product is treated with an emulsion of an organic liquid in water, optionally in the presence of a surfactant.
U.S. Pat. No. 5,175,282 also discloses a process for preparing beta-phase copper phthalocyanine pigment. Crude phthalocyanine is ball milled in the presence of organic solvents at 100.degree. to 120.degree. C. and then recrystallized at 90.degree. to 140.degree. C. in water mixed with 1 to 10% of an organic solvent.
The object of the present invention is to provide an improved process for preparing beta-phase metal phthalocyanine pigments having excellent dispersibility, tinting strength, clearness, gloss, and stability in coating compositions, printing inks, plastics, and the like. The present invention does not use special additives but nevertheless provides an economical route to products that are substantially 100% beta-phase pigments and that, in comparison with beta-phase copper phthalocyanines prepared by known methods, exhibit at least comparable dispersibility, brilliancy, and heat stability in plastics, inks, and paints.